The chemokines are a family of structurally related, heparin-binding basic small proteins of 8-14 kDa. Functionally, they can be classified as proinflammatory, homeostatic, or dual function (Moser, Wolf et al. 2004). Inflammatory chemokines are induced by pathogens, cytokines, or growth factors and recruit effector leukocytes to sites of infection, inflammation, tissue injury, and tumor. Such chemokines regulate the recruitment, activation, and proliferation of white blood cells (leukocytes) (Schall and Bacon 1994; Springer 1995; Baggiolini 1998). Chemokines selectively induce chemotaxis of neutrophils, eosinophils, basophils, monocytes, macrophages, mast cells, T and B cells. In addition to their chemotactic effect, they can selectively exert other effects in responsive cells like changes in cell shape, transient increase in the concentration of free intracellular calcium ions, degranulation, upregulation of integrins, formation of bioactive lipids (leukotrienes, prostaglandins, thromboxane), or respiratory burst (release of reactive oxygen species for destruction of pathogenic organisms or tumor cells). Thus, by provoking the release of further proinflammatory mediators, chemotaxis and extravasation of leukocytes towards sites of infection or inflammation, chemokines trigger escalation of the inflammatory response. Homeostatic chemokines, on the other hand, are expressed predominantly in bone marrow and lymphoid tissues and are involved in hematopoiesis, immune surveillance, and adaptive immune responses (Godessart 2005).
Based on the arrangement of the first two of four conserved cysteine residues, the chemokines are divided into four classes: CC or β-chemokines (e.g.) in which the cysteines are in tandem, CXC or α-chemokines, where they are separated by one additional amino acid residue, XC or γ chemokines (lymphotactin/XCL1 as only representative to date) that possess only one disulfide bridge, and CX3C-chemokines which feature three amino acid residues between the cysteines (membrane-bound fractalkin as only class member; (Bazan, Bacon et al. 1997)).
The CXC chemokines act primarily on neutrophils, in particular those CXC chemokines that carry the amino acid sequence ELR on their amino terminus. Examples of CXC chemokines that are active on neutrophils are IL-8/CXCL8, GROα/CXCL1, GROβ/CXCL2, and GROγ/CXCL3, NAP-2/CXCL7, ENA-78/CXCL5, SDF-1/CXCL12 and GCP-2/CXCL6. The CC chemokines act on a larger variety of leukocytes, such as monocytes, macrophages, eosinophils, basophils, as well as T and B lymphocytes (Oppenheim, Zachariae et al. 1991; Miller and Krangel 1992; Baggiolini, Dewald et al. 1994; Jose, Griffiths-Johnson et al. 1994; Ponath, Qin et al. 1996). Examples of these are I-309/CCL1; MCP-1/CCL2, MCP-2/CCL8, MCP-3/CCL7, MCP-4/CCL13, MIP-1α/CCL3 and MIP-1β/CCL4, RANTES/CCL5, and cotaxin/CCL11.
Chemokines act through receptors that belong to a superfamily of seven transmembrane-spanning G protein-coupled receptors (GPCRs; (Murphy, Baggiolini et al. 2000)). Generally speaking, chemokine and chemokine receptor interactions tend to be promiscuous in that one chemokine can bind many chemokine receptors and conversely a single chemokine receptor can interact with several chemokines Some known receptors for the CXC chemokines include CXCR1, which binds GROα, GCP-2, and IL-8; CXCR2, which binds chemokines including GROα, GROβ, GROγ, ENA-78, and IL-8; CXCR3, which binds chemokines including PF4, MIG, IP-10, and I-TAC; CXCR4 which thus far has been found only to signal in response to SDF-1, and CXCR5, which has been shown to signal in response to BCA-1 (Godessart 2005).
SDF-1 (stromal-cell derived factor-1; synonyms, CXCL12; PBSF [pre-B-cell growth-stimulating factor]; TPAR-1 [TPA repressed gene 1]; SCYB12; TLSF [thymic lymphoma cell stimulating factor]; hIRH [human intercrine reduced in hepatomas]) is an angiogenic CXC chemokine that does not contain the ELR motif typical of the IL-8-like chemokines (Salcedo, Wasserman et al. 1999; Salcedo and Oppenheim 2003) that binds and activates the G-protein coupled receptor CXCR4. The chemokine was discovered by three groups independently, either by cloning cDNAs that carry N-terminal signal sequences (Tashiro, Tada et al. 1993), by virtue of its ability to stimulate early B cell progenitors when expressed by the stromal cell line PA6 (Nagasawa, Kikutani et al. 1994), or by isolation from a cDNA library constructed from mouse embryo fibroblasts treated with the protein kinase C-activator tetra dodecanoyl phorbol acetate (TPA) (Jiang, Zhou et al. 1994).
As a result of alternative splicing, there are two forms of SDF-1, SDF-1α (68 AA) and SDF-1β, which carries four additional residues at the C-terminus (Shirozu, Nakano et al. 1995). The biological significance of these two splice variants is not completely understood.
The sequence conservation between SDF-1 from different species is remarkable: human SDF-1α (SEQ ID NO:1) and murine SDF-1α (SEQ ID NO:2) are virtually identical. There is a only a single conservative change of V to I at position 18 (Shirozu, Nakano et al. 1995). Another unusual feature that distinguishes SDF-1 from most other chemokines is its selectivity. In fact, SDF-1 and the receptor CXCR4 seem to comprise a monogamous receptor-ligand pair.
An NMR structure model exists (PDB access, 1SDF) for SDF-1 [8-68]. SDF-1 was found to be a monomer with a disordered N-terminal region. Differences from other chemokines are found mainly in the packing of the hydrophobic core and surface charge distribution (Crump, Gong et al. 1997).
Physiological activities of SDF-1: since the SDF-1 receptor CXCR4 is widely expressed on leukocytes, mature dendritic cells, endothelial cells, brain cells, and megakaryocytes, the activities of SDF-1 are pleiotropic. This chemokine, more than any other identified thus far, exhibits the widest range of biological functions, especially outside of the immune system. The most significant functional effects of SDF-1 are:
Homing and attachment of epithelial cells to neovascular sites in the choroid portion of the retina. SDF-1 has been shown to be involved in homing of epithelial cells to the choroid during neovascularization in eye tissue. The exact role of these cells is still under investigation but the published hypothesis is that epithelial cells are involved in the formation of aberrant blood vessels (Sengupta, Caballero et al. 2005);
Hematopoiesis.
SDF-1 is required to maintain hematopoietic progenitor (CD34+) cells in the bone marrow of the adult. AMD3100, a selective CXCR4 antagonist, can be used to mobilize CD34+ cells for hematopoietic stem cell transplantation. CD34+ cells migrate in vitro and in vivo towards a gradient of SDF-1 produced by stromal cells (Aiuti, Webb et al. 1997);
B Cell Development and Chemotaxis.
SDF-1 supports proliferation of pre-B-cells and augments the growth of bone marrow B cell progenitors (Nagasawa, Kikutani et al. 1994); it induces specific migration of pre-B cells and pro-B cells, while not acting as a significant chemoattractant for mature B cells (D'Apuzzo, Rolink et al. 1997; Bleul, Schultze et al. 1998). Presumably, SDF-1 is important for the positioning of B cells within secondary lymphoid tissue;
T Cell Chemotaxis.
SDF-1 is one of the most efficacious T cell chemoattractants; CXCR4 is present on many T cell subsets (Bleul, Farzan et al. 1996);
Embryonic Development.
SDF-1 and its receptor CXCR4 are essential for embryonic development. SDF-1 and CXCR4 knockout mice die perinatally; they exhibit cardiac ventricular septal defects or abnormal cerebellar development in addition to reduced numbers of B cell and myeloid progenitors (Nagasawa, Hirota et al. 1996; Ma, Jones et al. 1998; Zou, Kottmann et al. 1998). SDF-1 is also required for normal ontogeny of blood development during embryogenesis (Juarez and Bendall 2004); and
HIV Infection.
SDF-1 is able to inhibit T-tropic HIV-1 entry into CXCR4-bearing cell lines, and SDF-1 expression may have an important bearing on AIDS pathogenesis, since a polymorphism in the human SDF-1 gene affects the onset of AIDS (Bleul, Farzan et al. 1996).
Altered expression levels of SDF-1 or its receptor CXCR4 or altered responses towards those molecules are associated with many human diseases, such as retinopathy (Brooks, Caballero et al. 2004; Butler, Guthrie et al. 2005; Meleth, Agron et al. 2005); cancer of breast (Muller, Homey et al. 2001; Cabioglu, Sahin et al. 2005), ovary (Scotton, Wilson et al. 2002), pancreas (Koshiba, Hosotani et al. 2000), thyroid (Hwang, Chung et al. 2003), nasopharynx (Wang, Wu et al. 2005); glioma (Zhou, Larsen et al. 2002); neuroblastoma (Geminder, Sagi-Assif et al. 2001); B cell chronic lymphocytic leukemia (Burger, Tsukada et al. 2000); WHIM syndrome (warts, hypogammaglobulinemia, infections, myelokathexis) (Gulino, Moratto et al. 2004; Balabanian, Lagane et al. 2005; Kawai, Choi et al. 2005); immunologic deficiency syndromes (Arya, Ginsberg et al. 1999; Marechal, Arenzana-Seisdedos et al. 1999; Soriano, Martinez et al. 2002); pathologic neovascularization (Salvucci, Yao et al. 2002; Yamaguchi, Kusano et al. 2003; Grunewald, Avraham et al. 2006); inflammation (Murdoch 2000; Fedyk, Jones et al. 2001; Wang, Guan et al. 2001); multiple sclerosis (Krumbholz, Theil et al. 2006); rheumatoid arthritis/osteoarthritis (Buckley, Amft et al. 2000; Kanbe, Takagishi et al. 2002; Grassi, Cristino et al. 2004).
In experimental animal settings, antagonists of SDF-1 or its receptor have proved efficient for blocking growth and/or metastatic spreading of human cancer cells of different origin, such as pancreas (Guleng, Tateishi et al. 2005; Saur, Seidler et al. 2005), colon (Zeelenberg, Ruuls-Van Stalle et al. 2003; Guleng, Tateishi et al. 2005), breast (Muller, Homey et al. 2001; Lapteva, Yang et al. 2005), lung (Phillips, Burdick et al. 2003), glioblastoma/medulloblastoma (Rubin, Kung et al. 2003), prostate (Sun, Schneider et al. 2005), osteosarcoma (Perissinotto, Cavalloni et al. 2005), melanoma (Takenaga, Tamamura et al. 2004), stomach (Yasumoto, Koizumi et al. 2006) and multiple myeloma (Menu, Asosingh et al. 2006).
In addition, anti-SDF-1 therapy was beneficial in animal models in preventing retinal neovascularization (Butler, Guthrie et al. 2005), nephritis (Balabanian, Couderc et al. 2003) and arthritis (Matthys, Hatse et al. 2001; Tamamura, Fujisawa et al. 2004; De Klerck, Geboes et al. 2005).
SDF-1 is a player in the pathology of diseases of the back of the eye such as diabetic retinopathy (DR) (Fong, Aiello et al. 2004) and age-related macular degeneration (AMD) (Ambati, Anand et al. 2003). Both of these diseases damage the eye and lead to gradual loss of vision culminating in blindness. The damage occurs due to the inappropriate growth of blood vessels in the back of the eye, a process known as choroidal neovascularization (CNV). During CNV, new blood vessels that originate from the choroid migrate through a break in the Bruch membrane into the sub-retinal pigment epithelium (sub-RPE) or subretinal space. The abnormal vessels can bleed (intraretinal hemorrhage) or leak fluid under the retina. This can leave scars and can elevate the macula, which distorts vision.
SDF-1 is thought to play a role in CN V via recruitment of endothelial precursor cells (EPCs) to the eye. These precursor cells then become key structural components in the aberrant blood vessels.
Diabetic retinopathy is a major sequel to diabetes, occurring frequently in patients with both type 1 and type 2 diabetes. There are approximately 16 million diabetics in the U.S., with nearly 8 million having some form of diabetic retinopathy. When proliferative diabetic retinopathy (PDR) is left untreated, about 60% of patients become blind in one or both eyes within 5 years. With the alarming rise in the prevalence of diabetes in North America, Europe and many emerging countries, the patient population is growing quickly. For instance, the incidence of blindness is 25 times higher in patients with diabetes than in the general population. Furthermore, diabetic retinopathy (DR) is the most common cause of blindness in middle-aged subjects, accounting for at least 12 percent of all new cases in the United States each year. Screening programs are in place so that the vision of diabetes patients can be monitored and treatment, such as is available, can be delivered in time.
The direct causes of diabetic retinopathy are poorly understood, but the disease is thought to have its origins in a combination of sources: impaired auto-regulation of retinal blood flow; accumulation of sorbitol inside retinal cells; and accumulation of advanced glycosylation end products in the extracellular fluid. All of these factors are related directly or indirectly to hyperglycemia, the abundance of sugar in the bloodstream.
The symptoms of DR are similar to those of AMD. Patients lose cells in the retina and microaneurysms (blood flows) occur in the basement membrane of the retina. In addition, VEGF, IGF-1 and other blood-borne factors, possibly including SDF-1, attract new vascular cells and encourage the formation of damaging blood vessels.
Age-related macular degeneration (AMD) destroys a person's central vision. The early stages of the disease may not even be noticeable, because symptoms vary among patients. Sometimes a patient is affected only in one eye, or vision may be impaired in both eyes but not significantly. The disease causes distortion or faulty color perception. There is often a dark spot in the center of the visual field.
The etiology (course) of the disease is poorly understood. AMD is often thought of as the aging of the outermost layer of the retina. The physical alterations occur in the center of the retina, also known as the macula, which is the part of the retina relied upon for the most acute vision.
Wet AMD begins as a sequel to the dry form of the disease. Some 90% of patients suffer from the dry form of AMD, which results in the thinning of macular tissues and disturbances in its pigmentation. The rest have the wet form, which involves the bleeding described above.
The wet form of AMD represents an ideal market for a novel therapeutic: already the most common cause of blindness in people over the age of 55, AMD afflicts an estimated 4% to 5% of the United States population aged 65-74 and nearly 10% of those 75 years of age or older. There are already 5 million people in the United States alone over the age of 80 who have this disease and another 5 million people are expected to be affected by 2020.
Tumors are not just masses of cancer cells: infiltration of tumors with immune cells is a characteristic of cancer. Many human cancers have a complex chemokine network that influences the extent and phenotype of this infiltrate, as well as tumor growth, survival and migration, and angiogenesis. Most solid tumors contain many non-malignant stromal cells. Indeed, stromal cells sometimes outnumber cancer cells. The predominant stromal cells that are found in cancers are macrophages, lymphocytes, endothelial cells and fibroblasts.
Malignant cells from different cancer types have different profiles of chemokine-receptor expression, but the SDF-1 receptor CXCR4 is most commonly found in mouse and man. Tumor cells from at least 23 different types of human cancers of epithelial, mesenchymal, and haematopoietic origin express CXCR4 (Balkwill 2004). SDF-1 is the only known ligand for CXCR4. Apart from the bone marrow and secondary lymphoid tissue, where it is constitutively expressed, SDF-1 is found in primary tumor sites in lymphoma (Corcione, Ottonello et al. 2000) and brain tumors of both neuronal and astrocytic lineage. Furthermore, it is present at high levels in ovarian (Scotton, Wilson et al. 2002) and pancreatic cancer (Koshiba, Hosotani et al. 2000) as well as at sites of metastasis in breast (Muller, Homey et al. 2001) and thyroid cancer (Hwang, Chung et al. 2003), neuroblastoma and haematological malignancies (Geminder, Sagi-Assif et al. 2001). In contrast, CXCR4 expression is low or absent on normal breast (Muller, Homey et al. 2001), ovarian (Scotton, Wilson et al. 2002) and prostate epithelia (Sun, Schneider et al. 2005). CXCR4 expression thus seems to be a general characteristic of the malignant epithelial cell and not its normal counterpart.
Inhibiting chemokine-receptor signalling on tumor cells has the potential to induce growth arrest or apoptosis, and prevent invasion and metastasis in vivo.
CXCR4 knockdown by siRNA abrogated breast tumor growth (Lapteva, Yang et al. 2005); T-hybridoma cells which were transfected with a construct that prevents surface expression of CXCR4 could no longer metastasize to distant organs when injected intravenously into mice (Zeelenberg, Ruuls-Van Stalle et al. 2001); in similar experiments with colorectal cancer cells, lung and liver metastases were greatly reduced (Zeelenberg, Ruuls-Van Stalle et al. 2003); anti-CXCR4 antibodies inhibited the spread of breast cancer xenografts to the lymph nodes (Muller, Homey et al. 2001); treatment of lymphoblastoid cells with anti-CXCR4 or anti-SDF-1 antibodies delayed tumor growth in (NOD)/SCID mice (Bertolini, Dell'Agnola et al. 2002); anti-SDF-1 antibodies inhibited development of organ metastases of non-small-cell lung cancer (NSCLC) cells (Phillips, Burdick et al. 2003); systemic administration of the CXCR4 antagonist AMD3100 (AnorMED) inhibited the growth of intracranial glioblastoma and medulloblastoma xenografts, and increased tumor cell apoptosis within 24 hours (Rubin, Kung et al. 2003); anti-SDF-1 antibodies inhibited growth of MCF-7 breast cancer cells admixed with carcinoma-associated fibroblasts (Orimo, Gupta et al. 2005); neutralization of CXCR4 with antibodies blocked prostate cancer metastasis and growth in osseous sites (Sun, Schneider et al. 2005); and development of lung metastasis after injection of osteosarcoma cells was prevented by administration of the peptidic CXCR4 antagonist T134 (Perissinotto, Cavalloni et al. 2005).
Different authors come to the conclusion that targeting the SDF-1/CXCR4 axis may provide new therapeutic options for cancer patients:
Human ovarian tumors strongly express SDF-1 plus, on a lower level, VEGF. Both proteins are triggered by hypoxia in the tumor. Pathologic concentrations of any of the proteins alone were not sufficient to induce in vivo angiogenesis, but together, SDF-1 and VEGF in pathologic concentrations efficiently and synergistically induced neovascularization. Thus, interrupting this synergistic axis, rather than VEGF alone, can be a novel efficient antiangiogenesis strategy to treat cancer (Kryczek, Lange et al. 2005);
Breast cancer cell lines, when equipped with the autocrine SDF-1/CXCR4 signalling pathway, display aggressive behavior. This includes an increase in invasiveness and migration together with faster growth. The SDF-1/CXCR4 axis may thus provide important information for predicting the aggressive nature and constitute important therapeutic targets in human breast cancer (Kang, Watkins et al. 2005);
Migration and metastasis of small cell lung cancer (SCLC) cells—which express high levels of CXCR4—are regulated by SDF-1. Activation of CXCR4 promotes adhesion to accessory cells (such as stromal cells) and extracellular matrix molecules within the tumor microenvironment. These adhesive interactions result in an increased resistance of SCLC cells to chemotherapy. As such, inhibitors of the SDF-1/CXCR4 axis may increase the chemosensitivity of SCLC cells and lead to new therapeutic avenues for patients with SCLC (Hartmann, Burger et al. 2004); and
The SDF-1/CXCR4 axis emerges as a pivotal regulator of trafficking of various types of stem cells in the body. Since most if not all malignancies originate in the stem/progenitor cell compartment, cancer stem cells also express CXCR4 on their surface and, as a result, the SDF-1/CXCR4 axis is involved in directing their trafficking/metastasis to organs that express SDF-1 (e.g. lymph nodes, lungs, liver and bone). In consequence, strategies aimed at modulating the SDF-1/CXCR4 axis could have important clinical applications both in regenerative medicine to deliver normal stem cells to the tissues and in clinical oncology to inhibit metastasis of cancer stem cells (Kucia, Reca et al. 2005).